In an electronically commutated motor (ECM), it is known to control the rotation speed by modifying the length of current blocks chat are supplied to the stator winding arrangement of that motor. This is referred to as “block control.”
At low rotation speeds, the power demand of an ECM is usually low, and the current blocks are then short and have a high amplitude, since the induced voltage of the motor is low at low rotation speeds. The motor current is then essentially pulsed-shaped, and that results in undesirable motor noise. Conversely, the current blocks at high rotation speeds are long, and their amplitude decreases because of the high induced voltage, which can result in commutation problems.